Oil Separator


Oil Removal System Separator for light liquids in stormwater treatment

Category / type: stormwater, car washing, light liquids, mineral greases and oil.
Application: service areas, parking lot, petrol stations, car wash, junkyard.
Nominal Size (NS): 1,5 3, 6, 10, 20, 30, 40, 50, 65, 80, 100, 125, 150, 200, 300, 400, , 500.
Normative references: EN 858-1 e 858-2, local and regional regulations.
Material: concrete, polyethylene, polypropylene, glass-reinforced plastic (GRP).
Upgradable: light liquid sensor, rain sensor, flow meter, oil skimmer.

The oil and light liquid separator (e.g. petrol, diesel) is a watertight tank built to treat run-off water and to reduce pollutant concentration in the effluent.
Water coming from the paved surfaces, warehouses, parking, garage, car washes and gas stations usually contains lubricants, mineral oils, light liquid as fuels that must be removed before the drainage into receiving water or onto ground.
The oil separator can be made by different material depending on technical requirements and it works by gravity or coalescence process. Light liquids move towards water surface, where they are stored and periodically removed by cleaning operation.
Oil and light liquid separator works on rainwater pipeline upstream from public sewers as often required by drainage system managing authority.
The main function is to avoid oil and grease problems in sewer and to protect environment from spills of dangerous pollutants such as hydrocarbons.
Light liquids are very often not biodegradable and they can create problems in wastewater treatments and in water courses.
It works according to specific weight difference between water and light liquids that can be stored on water surface.
Flotation can be improved by coalescence: passing through filters oil drops merge together increasing their size and therefore their climb rate.
A good efficiency is possible with regular cleaning operations as well as with an appropriate sizing.
Maintenance should be carried out by skilled and trained personnel from authorized and specialized firms. The pollutant removed must be transferred to an appropriate treatment systems.
Oil separator must be always kept full of water up to working level in order to give the surface for oil floatation.
Baffles are placed to avoid light liquid and scum dragging.
The separator may be provided with coalescing filters, an automatic closure device and warnings system.
Oil separator is often installed with a sand trap installed upstream to meet Europe requirement (sludge trap).
It treats continuous flow rate up to a project value; flows above this rate are allowed to bypass the separator.
The oil separator is able to treat only run-off water from impervious surfaces and it’s not allowed to pour used engine oil coming from vehicle maintenance (intentional disposal of oil in the tank is absolutely forbidden).


How It Works

The oil and light liquid separator (e.g. petrol, diesel) is a watertight tank built to treat run-off water and to reduce pollutant concentration in the effluent.
Water coming from the paved surfaces, warehouses, parking, garage, car washes and gas stations usually contains lubricants, mineral oils, light liquid as fuels that must be removed before the drainage into receiving water or onto ground.
According to European regulation there are two different kind of oil separator sizing: the first referees to the flow rate, that can be treated by a separator (tank), wile the second one calculates the project flow for each different users (rain, carwash, etc.).
Physical sizing calculates the treatment flow (in litre per second or cubic meter per hour) and a maximum oil concentration for each separator depending on inlet oil concentration and the minimum oil drop diameter to hold.
The minimum required area is calculated according to the Stokes’ law. Coalescence filter oil separator is similar to lamellar pack settlements: it provides a greater surface area for the same volume. Additional filters can improve treatment efficiency.
Inlet and outlet parameters are set by European legislation. See regulatory framework chapter.
The separator main parameter is nominal size (NS) and the class (I or II).
Maximum stormwater flow is calculated by hydrological analysis depending on the project return period.
Pollutants are calculated according to experience, to literature data or by chemical analysis.
For more information see car wash solution.
The European standard defines criteria and specific flows for each element.
It refers to EN 752-4 (Drain and sewer systems outside buildings. Hydraulic design and environmental considerations) to calculate the rain flow and two dimensionless parameters for NS calculation: impediment factor depending on the nature of the discharge fx ( and density factor for the relevant light liquid fd ( – Appendix A).
European law allows the combined sewage discharge and, if they are not simultaneous, only maximum value should be considered (4.3.3).
Since in the separator no biological reactions are present, there’s no oversizing problems; on the contrary it allows to decrease the cleaning operation frequency.
Ventilation is needed or by sewer pipes and or by vent directly connected to the separator.


Operation and maintenance

Separator works by gravity effect an so it don’t need start up procedure; tank must be filled with clean water up to the normal operating level before activation.
Oil level must be regularly monitored, by visual contact or by light liquid sensor, and when storage volume is full a purging is request.
Higher volume causes the hydraulics residence time reduction and worsen the treatment efficiency.
Maintenance and cleaning operation must be done by licensed firms, able to follow the procedures for the correct disposal of hazardous materials.
After light liquids removal, tank is filled again with clean water up to working level.


Regulatory framework

The European standard EN 858-1 describes “principles of product design, performance and testing, marking and quality control” and defines separator nominal size (NS) (3.7).
It standardizes the nominal flow size concept and it refers to a concentration of a specific light liquid (specific weight known) and defines testing methods for NS designation.
Norm, except for minimum inlet and outlet pipe diameter, does not refer to geometric parameters but it requires structural stability (6.4) and oil concentration (inlet and outlet) for testing procedure.
There are two classes of separators based on performance under standard test conditions: class I for residual content lower than 5 mg/l (recommended coalescing filter) and class II for residual content lower than 100 mg/l according to essay standard EN 858.
This latter clearly describe how should be the CE marking and test method: for example the reaction to fire must be checked according to clause 8.4 in EN 858-1.
Dimensional parameter shown on table 3 refers to a separator equal or greater than NS 150 (NS) that can be built on site.
Separators need a Declaration of Performance (DoP) based on harmonized standard according to Regulation (EU) No 305/2011.
The European standard 858-2 “(…Selection of nominal size, installation, operation and maintenance”) explains how to choose the nominal size depending on users, components of separator systems, their combination and application.
Minimum sludge trap volume are listed in table 5. Sludge trap is often required upstream to separator.
The light liquid separator is subject to prior authorisation by competent authorities.
Some local or national regulations, require the use of separators, for constant flow rate or first rain water treatment.


Features and benefits

Oil and light liquids separator is used in run-off water treatment from parking place, roads, industrial sites where oil is stored or used, refuelling facilities, car wash (as treatment component) and from any other site with a risk of oil contamination.
It is placed on rainwater drainage line and often with sand or sludge trap upstream and filters or physical chemical plants downstream.
Local regulations prohibit to discharge into ground and/or watercourses pollutants such as hydrocarbons (oils, greases, fuel etc.). The discharge into the sewer system requires separators or additional treatment plants.
It is also used as part of more complex treatment as first rain plants, continuous rain flow plants and car wash solutions (see in specific sections).

Advantages (benefits)

  • simply treatment;
  • no energy consumption and supply;
  • small area required (underground installation);
  • low operating costs;
  • long useful life.


  • low efficiency (further treatment may be required);
  • periodic purges, depending on use, by a licensed contractor (vacuum truck).


Products in category Oil Separator